1. Field of the Invention
The present invention relates to a circuit and a method for driving an organic light emitting display. More particularly, the present invention relates to a circuit and a method for driving pixels of an organic light emitting display.
2. Description of the Related Art
Organic light-emitting displays based on organic light-emitting diodes have many advantages, such as spontaneous light emission, high luminance, high contrast, wide viewing angle and fast response. Therefore, scientists and engineers have been making a lot of effort on research and development of characteristics of and driving circuits for organic light-emitting displays. However, although organic light-emitting displays have the advantages mentioned above, there are still some problems waiting to be solved.
FIG. 1 depicts a basic circuit for driving the organic light-emitting diode OLED that is part of a pixel of an organic light-emitting display. When the thin-film transistor (TFT) T2 connected to the scan line SL is turned on, a data voltage is stored into the storage capacitor Cs. And then the data voltage stored in the storage capacitor Cs determines the current passing through the TFT T1, and thereby determines the brightness of the organic light-emitting diode OLED. This driving circuit is simple. However, it has some problems such as threshold voltage shift and shortened material lifetime of organic light-emitting diodes.
Drifting threshold voltage means that the threshold voltages of driving switches tend to vary because of factors such as time and fabrication process. The current through organic light-emitting diodes also tends to vary according to the drifting. Consequently, the brightness of pixels of an organic light-emitting display is often discordant even when the pixels receive identical data signals. For solving this problem, the article by H. Kageyama et. al. and titled “A 2.5-inch OLED Display with a Three-TFT Pixel Circuit for Clamped Inverter Driving” (SID2004) proposed the circuit depicted in FIG. 2. The circuit in FIG. 2 clamps and stores the threshold voltage factor (VDD−Vth, where Vth is the threshold voltage of the TFT T1) into the storage capacitor Cs by switching the TFT T2 and T3. Later, during the period with an external electric field of the organic light-emitting diode OLED, the voltage stored in the storage capacitor Cs will cancel out the threshold voltage of the TFT T1. In this way, the problem of discordant brightness caused by threshold voltage shift is solved.
About material lifetime of organic light-emitting diodes. The article by Dechun Zou et. al. and titled “Improvement of Current-Voltage Characteristics in Organic Light Emitting Diodes by Application of Reversed-Bias Voltage” (Japanese Journal of Applied Physics, vol. 37, pp. L1406–L1408, 1998) disclosed the polarization phenomenon induced during the period with an external electric field of organic light-emitting diodes. Please refer to FIG. 3 and FIG. 4. FIG. 3 shows the random distribution of ionic impurities inside an organic light-emitting diode during its period without an external electric field (that is, when the diode does not emit light), while FIG. 4 shows the distribution of the ionic impurities during the period with an external electric field of the diode. In the period with an external electric field, the external electric field E across the organic light-emitting diode separates positive charges and negative charges in the ionic impurities. Therefore the internal reverse electric field R is generated in response to the external electric field E. This is the polarization phenomenon. The polarization phenomenon not only shortens material lifetime of organic light-emitting diodes, but also hinders the movement of electrons and holes inside the diodes and reduces the light-emitting efficiency of the diodes.
Against the polarization phenomenon, the article by Si Yujuan et. al. and titled “A Simple and Effective AC Pixel Driving Circuit for Active Matrix OLED” (IEEE Transactions on Electron Devices, vol. 50, issue 4, pp. 1137–1141, April 2003) proposed the circuit depicted in FIG. 5. The voltage source Vref in FIG. 5 switches between 0V and a high voltage so that the organic light-emitting diode OLED is reverse-biased periodically. The reverse bias serves to join the separated positive and negative charges to eliminate the polarization phenomenon. Therefore the circuit in FIG. 5 is capable of prolonging the material lifetime of organic light-emitting diodes and enhancing the movement of electrons and holes inside the diodes.
As can be seen from the above, so far the prior art can solve only one of the polarization phenomenon and the problem of threshold voltage shift. One of the goals of the present invention is solving the polarization phenomenon and the problem of threshold voltage shift at the same time.